1. Field of the Invention
The present invention generally concerns amplifier design and amplifier operation, particularly for wireless cellular radio communications applications where occasional jamming is prevalent.
The present invention particularly concerns the realization by both design and operation of a Low Noise Amplifiers (LNA) simultaneously improved in (i) dynamic range and (ii) overall power consumption, these seemingly contradictory requirements being satisfied by optimizing power consumption in the LNA in consideration of its instant operating environment.
The present invention further particularly concerns an ungrounded power detector that is both fast and sensitive to detect the output power of, for example, a LNA.
2. Description of the Prior Art
2.1 Low Noise Amplifiers, and Amplifier Operation in the Presence of Jamming
With the explosive growth of wireless communications, the airwaves are rapidly being filled with signals of varying strengths and frequencies. Immunity to jamming has subsequently become a significant concern to any communication system. This is especially true for a mobile communication system, such as a cellular phone, as it is difficult to predict the jamming environment the system will be exposed to. At the same time, the need for portability, and thus long battery life, requires the system to consume as little power as possible.
In a typical wireless system, filtering before the low-noise amplifier can reject most jammers. However, a high rejection ratio incurs high insertion lossxe2x80x94a direct contributor of receiver sensitivity degradation. In addition, many close-in jammers are impossible to block given the size and cost restrictions of a mobile system. A number of different jammers including frequency modulation radio, television, navigational beacons, and microwave ovens will typically be detected by an omni-directional 2.5 GHz antenna. The low-noise amplifier, therefore, must have a large dynamic range: namely, a low noise figure and low intermodulation distortion. See S. Chen, xe2x80x9cLinearity Requirements for Digital Wireless Communi-cation,xe2x80x9d IEEE GaAs IC Symp. Dig., Anaheim, Calif., pp. 29-32, October 1997.
To meet these demands, the LNAs often consume the most power in a receiver; tradeoffs are usually required to balance dynamic range versus power consumption.
2. Power Detector
Power detector circuits are many and various, and are not commonly identified as requiring improvement. The low noise amplifier circuit of the present invention will show, however, that it is useful (but not necessary) to detect instantaneous amplifier output power, or (equivalently) voltage (into load), with two orders of magnitude (i.e., xc3x97100) greater sensitivity that existing Schottky diode power detectors. To this end the present invention will be found to encompass a power detection circuit that is particularly characterized in that the power is not detected relative to ground, ergo an un-grounded power detection circuit. When a signal in which power is detected need not be sunk to, nor referenced relative to, ground, then it becomes possible to detect variations in the signal with much greater sensitivity.
The present invention contemplates a Low Noise Amplifier (LNA) that circumvents the compromise between (i) dynamic range and (ii) power consumption by optimizing power consumption for the operating environment. The LNA of the present invention exhibits a high dynamic range when it is near or in compression, but low power consumption when it is in small-signal operation where a large dynamic range is not necessary.
Furthermore, the dynamic range of the amplifier is extended: jamming may be countenanced without such distortion as would otherwise occur.
The present invention further contemplates an un-grounded a.c. signal power detection circuit that is very sensitive and very fast. This un-grounded power detection circuit will prove useful, even if not absolutely essential, in an s-band low-noise amplifier that is, in accordance with the present invention, improved for both power consumption and dynamic range, especially as both are required in a mobile environment.
1. A Method of Operating an Amplifier
In one of its aspects the present invention is embodied in a method of operating an amplifier where the amplifierxe2x80x94or, more exactly, the transistor components of the amplifierxe2x80x94has an load line. The amplifier is operated so as to emulate the property of a class AB amplifier where increasing amplifier input current raises the d.c. bias of the amplifier and increases amplifier output current. The amplifier is so operated nonetheless that it will never enter into class AB operation and will always operate in class A.
The method of operating an amplifier always in class A nonetheless to producing more output current from more input current includes two steps: 1) The amplified output of the class A amplifier is monitored; and, in response to detecting an increase in the amplifier output, 2) the load line of the amplifier is dynamically biasing to a higher d.c. bias point, causing the amplifier to consume more power and to produce a still larger amplified output signal. This xe2x80x9cboostingxe2x80x9d of the amplifier output could obviously cause a run-away condition, but this xe2x80x9cboostingxe2x80x9d is realized, in accordance with the present invention, so as to always maintain the amplifier to operate in class A.
The purpose of so operating a class A amplifier is demonstrated when the amplifier is used, inter alia, as an initial low-noise radio signal amplifier in a wireless communication system. In this environment an increase in amplifier output signal is indicative of a presence of a strong jammer component in the amplifier input signal. Moving the load line of the amplifier will cause the amplifier to draw more current, beneficially decreasing a noise figure while increasing gain of the amplifier. The amplifier will ultimately be caused to reach a new steady state with higher power and improved linearity. This improved response comes, of course, at the cost of increased power consumption,
Conversely, if no increase in amplifier output signal is detected then this is indicative that no strong jammer component is present within the amplifier input signal. In such a case neither the d.c. bias, nor the load line, will be raised, and the amplifier will operate quiescently, conserving power.
2. An Amplifier of Improved Dynamic Range
In another of its aspects, the present invention can be considered to be embodied in an amplifier of improved dynamic range.
The amplifier includes at least one Field Effect Transistor (FET) receiving at its gate an input signal from an external source, and amplifying this input signal in accordance with its drain-source bias voltage VDS to produce at its drain an amplified output signal.
A power detector circuit monitors the amplified output signal to produce a detected-power voltage signal VDD.
A dynamic bias control circuit compares the detected-power signal VDD to the drain-source bias voltage VDS so as to vary a gate-to-source voltage bias VGS of the input signal, actively moving a load line of the FET so as to cause the FET to consume more power when the amplified output signal becomes large.
The amplified output signal typically so becomes large because of a presence of a strong jammer component of the input signal. In this eventuality moving the load line of the at least one FET will cause the FET to draw more current, beneficially decreasing noise figure while increasing gain. Ultimately the amplifier of which the at least one FET forms a part to reach a new steady state with higher power and improved linearity.
However, when no strong jammer component of the input signal is present, and when the amplified output signal is correspondingly not large, then the FET, and the amplifier of which it forms a part, will remain biased in an operational condition where power is conserved.
Accordingly, the self-adjusting bias of the at least one FET results in both (i) improved power consumption and (ii) improved dynamic range in an environment where exists occasional strong jammer signals.
The at least one Field Effect Transistor (FET) preferably consists of two cascaded FETs, and more preferably consists of two cascaded FETs where each is a GaS FET. The first, input, one of the two cascaded FETs is most preferably a low-noise PHEMT while the second, output, one of the two cascaded FETs is most preferably a hetero-junction FET.
The dynamic bias control circuit preferably consists of two operational amplifiers each varying a gate-to-source voltage bias VGS of an associated FET.
The power detector circuit preferably consists of a resistor R, and a first diode D1 series connected to form a diode-limited resistive divider. This diode-limited resistive divider is preferably temperature compensated by a second diode D2, making that the power detector circuit of which it forms a part is also temperature compensated.
The amplifier is normally operational in S-band.
3. A Low-Noise Amplifier (LNA) Improved for Having an Elevated Third-Order Input Intercept Point (IP3) and a Reduced Noise Figure During Jamming
In yet another of its aspects, the present invention can be considered to be embodied in a low-noise amplifier (LNA) improved for having an elevated third-order input intercept point (IP3) and reduced noise figure during jamming.
The LNA includes (i) at least one active device amplifying in accordance with a bias signal an input signal received from an external source so as to produce an amplified output signal, (ii) a power detector monitoring the amplified output signal to produce a detected-power signal, and (iii) a dynamic bias control circuit responsive to any difference between the detected power signal and the bias signal to increase the bias signal. This increase in the bias signal actively moves a load line of the at least one active device so as to cause this device to consume more power when the amplified output signal is large.
By this operation, when the amplified output signal is large because of a presence of jamming then the moved load line of the at least one active device will cause the active device to draw more current, decreasing noise figure while increasing gain. The amplifier of which the at least one active device forms a part will be caused to reach a new steady state with higher power and improved linearity.
The power detector, the dynamic bias control circuit and the at least one active device preferably function in concert so that when no jamming is present then, at nominal small-signal conditions, the at least one active device will be biased to consume less power, conserving power in the amplifier of which it forms a part.
4. A Method of Low-Noise Amplification
In still yet another of its aspects, the present invention can be considered to be embodied in a method of low-noise amplification that is improved for (i) conserving power during nominal small-signal conditions, and also for (ii) increasing amplification gain, and reducing amplification noise figure, during input signal jamming, making less likely any loss of data.
The method consists of amplifying in at least one active devicexe2x80x94and in accordance with a bias signalxe2x80x94an input signal received from an external source so as to produce an amplified output signal.
This amplified output signal is monitored in a power detector to produce a detected-power signal.
Responsively to any difference between this detected power signal and the bias signal, the bias signal of the of at least one active device is adjusted in a dynamic bias control circuit so as to actively move a load line of this at least one active device. This movement of the load line causes this at least one active device to consume more power when the amplified output signal is large.
Accordingly, when the amplified output signal is large because of the presence of jamming (as in the cellular radio environment) the moved load line of the at least one active device will cause the active device to draw more current, decreasing noise figure while increasing gain. The amplifying will reach a new steady state with higher power and improved linearity.
However, when no jamming is present then, at nominal small-signal conditions, the at least one active device will be biased by the adjusting so as to consume less power, conserving power.
5. The Present Invention Expressed With Less Emphasis on Amplifier Power, and With More Emphasis on Amplifier Distortion
The previous explanations of the present invention, including that of the immediately previous section 4., have emphasized the situation-variable power consumption of a low-noise amplifier in accordance with the present invention, stating that the use of more power (more current) will, in the presence of jamming, serve to decrease the amplifier noise figure and distortion (while increasing gain). While a practitioner of amplifier design and amplifier operation will understand that 1) power and 2) distortion are xe2x80x9ctwo sides of the same coinxe2x80x9d, it may be useful for other persons less acquainted with the design and operation principles of low noise amplifiers to consider that the primary goal of the present invention is, after all, not to reduce power consumption but rather to reduce distortion, extending the operational range of the amplifier in the presence of jamming.
Emphasizing distortion, as opposed to power, reduction, it is therefore possible to perceive of the low-noise amplification method in accordance with the present invention as constituting four steps. An input signal received from an external source is amplified, in at least one active device and in accordance with a bias signal, so as to produce an amplified output signal. This amplified power signal is monitored in a power detector to produce a detected-power signal. This detected-power signal is compared with the bias signal to produce a difference signal. Finally, the bias signal of the at least one active device is adjusted, in and by a dynamic bias control circuit responsively to the difference signal, so as to (i) actively move a load line of this at least one active device until (ii) the difference signal becomes zero.
At this time such distortion as might otherwise have appeared in the amplified output signal will be minimized. This is because the moved load line will permit that a larger input may be handled by the amplifier without distortion. More exactly, when the amplified output signal is large because of a presence of a jamming signal then the moved load line of the at least one active device will permit that (i) a larger input resulting from combination of the jamming signal with the input signal will be amplified (ii) without such distortion as would otherwise occur in amplification of these combined signals should the load line have not been moved.
It will therefore be understood by a practitioner of amplifier design and amplifier operation that moving the load line in response to operational conditions benefits both the amplifier distortion and noise figure, and also the power consumption.
6. An Un-grounded Power Detector Circuit
In yet another of its aspects the present invention is embodied in a circuit for detecting a peak power of an a.c. signal. The circuit includes a resistive voltage divider, located between a voltage source and ground, that produces a reference voltage signal. A diode is connecting at its cathode to both the a.c. signal and to the reference voltage signal. Meanwhile an envelope detector is connected both to the anode of the diode and to the reference voltage. The output of the detector circuit appears across this envelope detector.
Circuit operation is such that the detector circuit output is equal to the reference voltage when the a.c. signal is zero. When the a.c. signal is not zero then the detector circuit output is, however, equal to a sum of (i) the reference voltage, and (ii) a voltage (equivalent to power) of an envelope of the a.c. signal.
The power detector circuit is notable in that power is detected without direct reference to ground. Instead, power is detected relative to a reference voltage, and across a single diode. Signal propagation across the diode is very fast, on the order of nanoseconds. Therefore the power detector circuit has a very fast response time. Because (i) the power within the a.c. signal is not compared to ground, but rather to an elevated voltage reference signal, and (ii) the voltage across the diode is much smaller than a conventional Schottky-diode-based power detector, the power detection is also very sensitive, on the order of microvolts. This combination of speed and sensitivity is useful in realizing the improved low-noise amplifiers of the present invention.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.